I have 3" ORY, Magnaflow hi flow cat, bullet/magnaflow with dual 24"x3" dumps and it is loud but does sound good.

 

It is a 5.3 stock bottom end stock heads.

 

if you're only talking about what to do after the muffler (and you already have single 3" before the muffler), then I don't think tailpipe size will make a difference. I would probably go with 2.5" just because it would probably be a little cheaper and be easier to fit.

 

To me, if you narrow the exhaust down to one small point anyway, whatever you do after that bottleneck can be just for looks. It's not much use trying to conserve the scavenging effect by going with smaller pipes, is what I'm trying to say.

To me, I'd keep it 3" all the way through SI/SO muffler. or I'd ditch the 3" Y-pipe and run 2.5" duals with an X or H-pipe. But I'm a simple man, even with a dual 2.75" x-piped exhaust, I still don't run tailpipes or have any of the exhaust appear out from under the truck.